Consolidated Statements of Comprehensive Income (Unaudited) - USD ($) shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Statement of Comprehensive Income [Abstract]
Operating revenues	$ 1,194.1	$ 1,139.0
Operating expenses:
Purchased transportation	473.3	425.0
Salaries, wages, and benefits	313.0	311.3
Fuel and fuel taxes	74.8	84.7
Depreciation and amortization	73.4	71.7
Operating supplies and expenses	145.1	119.1
Insurance and related expenses	28.2	23.1
Other general expenses	34.8	36.5
Total operating expenses	1,142.6	1,071.4
Income from operations	51.5	67.6
Other expenses (income):
Interest Income, Other	(2.2)	(0.7)
Interest expense	3.9	4.2
Other expenses (income)	0.4	(0.4)
Total other expense	2.1	3.1
Income before income taxes	49.4	64.5
Provision for income taxes	12.5	16.9
Net income	36.9	47.6
Other comprehensive income (loss):
Foreign currency translation adjustments	0.3	(0.4)
Unrealized gain (loss) on marketable securities—net of tax	0.4	(0.2)
Total other comprehensive income (loss)	0.7	(0.6)
Comprehensive income	$ 37.6	$ 47.0
Weighted average common shares outstanding	177.0	176.9
Earnings Per Share, Basic	$ 0.21	$ 0.27
Weighted average diluted shares outstanding (shares)	177.4	177.2
Diluted earnings per common share	$ 0.21	$ 0.27